3. Installing scrubbers in smokestacks and switching to
cleaner-burning fuel are the two methods available to Northern
Power for reducing harmful emissions from its plants. Scrubbers
will reduce harmful emissions more than cleaner-burning fuels will.
Therefore, by installing scrubbers, Northern Power will be doing
the most that can be done to reduce harmful emissions from its
plants.
Which of the following is an assumption on which the argument
depends?
(A) Switching to cleaner-burning fuel will not be more expensive
than installing scrubbers.
(B) Northern Power can choose from among various kinds of
scrubbers, some of which are more effective than others.
(C) Northern Power is not necessarily committed to reducing harmful
emissions from its plants.
(D) Harmful emissions from Northern Power’s plants cannot be
reduced more by using both methods together than by the
installation of scrubbers alone.（D）
(E) Aside from harmful emissions from the smokestacks of its
plants, the activities of Northern Power do not cause significant
air pollution.
为什么是D呢？这个为什么是假设？我选的是E！求指点！ | 
